Add CRL support to X.509 verification

[vEpiphyte]

Hello!

I've been working on updating some code to utilize cryptography in favor of PyOpenSSL due to the API deprecation in the older project.

The only code that I can not currently remove is related to the use of X509Store and X509StoreContext. That is utilized for doing certificate validation. For example:

    def verifyACertificate(self, cert: c_x509.Certificate):
    	'''
    	Check if a Certificate is trusted by a set of CAs and is not revoked.
    	'''
        crls = self.getCaCrls()  # type: List[c_x509.CertificateRevocationList]
        cacerts = self.getCaCerts()  # type: List[c_x509.Certificate]

        store = crypto.X509Store()  # This is pyopenssl...
        [store.add_cert(crypto.X509.from_cryptography(cacert)) for cacert in cacerts]

        if crls:
            # Setting flags is important. Other uses use things such as PARTIAL_CHAIN flags.
            store.set_flags(crypto.X509StoreFlags.CRL_CHECK | crypto.X509StoreFlags.CRL_CHECK_ALL)
            [store.add_crl(crypto.CRL.from_cryptography(crl)) for crl in crls]

        ctx = crypto.X509StoreContext(store, crypto.X509.from_cryptography(cert))
        try:
            ctx.verify_certificate()  # raises X509StoreContextError if unable to verify
        except crypto.X509StoreContextError as e:
            mesg = _unpackContextError(e)  # helper function to unpack the X509StoreContextError, not important here.  
            raise Exception(mesg)
        return cert

I believe my use case aligns with https://github.com/pyca/cryptography/issues/10276 ( doing code signing and/or user cert verification ). Current docs for the verification APIS ( https://cryptography.io/en/42.0.2/x509/verification/ ) don't seem to support setting CRLs or flag setting.

Is this type of use case in scope for work in https://github.com/pyca/cryptography/pull/10345 ?

[alex]

cc: @woodruffw

Looks like there's two pieces you need here:

1. Doing verification without checking for a particular SAN, which is what @woodruffw has in his PR
2. Checking CLRs.

Both of these are tracked in https://github.com/pyca/cryptography/issues/10034

Figuring out CLRs is going to require some API design work on our part. It looks like your implementation relies on having pre-fetched the CRLs, and not loading them on-demand.

[vEpiphyte]

@alex Correct - I am assuming that CRLs are present ahead of time ( and very much not assuming that it is the job of cryptography to retrieve them ).

If these pieces are being correctly tracked in #10034 we can close this issue out to avoid duplication.

[alex]

https://github.com/pyca/cryptography/pull/10345 adds verification without a subject.

CRL is the remaining piece here. We still need to figure out what we want to do in terms of API design there.